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AIM'KNDIX                                           26-2

using actual models, it will he found that they cannot be turned
so as to coincide until two of the groups in one model have been

The main difference between two such substances lies in their
action on polarised light, the one turning it to the right (dextro-
rotatory) and the other to the left (lacvo-rotatory), when in the
liquid or dissolved state. Although every optically active sub-
stance contains at least one asymmetric carbon atom like amyl
alcohol and malic acid, or two like tartaric acid (the asymmetric
carbon is represented in heavy type),



	no C--COOH

0    II
	no - c ....... ii

	no-c coon



aitiyl alcohol.
	Malic acid.
	Tnrtaric acid.

the converse does not always hold ; for there arc many com-
pounds which possess an asymmetric carbon atom and show no
rotation. The cause of this may be, either that the substance is
a. mixture of equal quantities of the two forms, which by having
opposite rotations neutralise each other's effect as in the case of
racemic acid, which consists of equal quantities of dcxtro- and
laevo-tartaric acid and produces what is termed "external com-
pensation,35 or the two similar asymmetric carbon atoms exist
within the same molecule and neutralise each other's effect by
"internal compensation," as in the case of mesotartaric acid.
External compensation is generally exhibited by artificially
prepared compounds as distinguished from natural products.
Thus, glyceric acid from glycerol is inactive, though it contains
an asymmetric carbon atom,

because it consists of a mixture of dcxtro- and lacvo-glyceric
acid in equal quantities, whereas tartaric acid, which occurs in
grapes, malic acid, which is obtained from mountain ash berries,
and also the sugars, terpenes, alkaloids, and a number of other